In the production of parts of composite materials such as graphite-epoxy composite parts for use in the aircraft and space industry, it is necessary to employ molds which are devoid of open porosity on the working surfaces. The reason for this requirement is essentially, two-fold, namely, (1) to prevent intrusion of liquid resin into the mold media and (2) to prevent gas infiltration through the mold when a high pressure differential is applied across the mold cross-section during the vacuum forming of the composite part.
Because of its superior dimensional stability over wide ranges of temperature variation and its high temperature tolerance, graphite is an ideal candidate for use as the mold forming material in the production of these composites. However, fine-grain graphite, as manufactured and machined, has undesirable surface porosity as well as dusting and poor wear characteristics which severely limit its use in this particular application.
A common method of eliminating porosity in graphite materials is to impregnate the graphite shape with a liquid resinous material which, when cured, fills or blocks virtually all of the open pores. In these impregnation processes, the graphite article is placed in an autoclave. The autoclave is evacuated to remove gases from the porous structure of the shape, and insure impregnation by the liquid resinous material. Generally, the electrode is then contacted with the resinous material under pressure to assist infiltration of the resin into the pores of the electrode. Following impregnation, the resin is cured by heating the impregnated electrodes. Multiple impregnation/curing cycles, usually three or more, are required to achieve complete sealing of porosity.
U.S. Pat. No. 2,909,450, issued to Goldstein on Oct. 20, 1959, discloses a method of treating a porous article, such as a carbon electrode, to eliminate its porosity. The article is impregnated with a solution containing 65-99% by weight of furfuryl alcohol, 0-25% by weight of furfural, 0.5-10% by weight of a zinc chloride catalyst and the balance water. The impregnation is carried out by immersing a carbon electrode in the solution within a treating cylinder and carbon electrode is subjected to a vacuum. Thereafter, the electrode, while immersed in the solution, is subjected to a pressure of 150 psi for one hour. The impregnated electrode is then removed from the treating cylinder and placed in an oven at 95.degree. C. to 100.degree. C. to cure for 18 to 24 hours. This procedure may be repeated one or more times until the porosity of the carbon electrode is essentially eliminated.
These prior art methods, however, suffer from the disadvantage in that they require treatment of the graphite articles under vacuum and then pressure in autoclaves or similar equipment. Certain precision machined graphite shapes and large parts are not easily processed in autoclave type equipment because of their configuration or size. Thus, these parts are difficult or impossible to treat by these prior-art methods. Another problem is that application of the resinous material to the part surface is uneven with some excess resinous material typically remaining upon the surface, forming a uneven coating with surface accumulations of resin. After curing of the resin, these surface accumulations and the uneven coating on the surface result in loss of the fine dimensional tolerances to which the graphite parts are machined.